Generally, a secondary battery can be recharged and discharged, unlike a primary battery. Researches on the secondary battery have been conducted to adapt the secondary battery to a high-technology field, such as digital cameras, cellular phones, notebook computers, or hybrid cars. As examples of secondary batteries, there are a nickel-cadmium battery, a nickel-metal hydride battery, a nickel-hydrogen battery, a rechargeable lithium battery, and others. Among them, the rechargeable lithium battery has an operating voltage of 3.6V or higher, and is used as a power source for portable electronic equipment. Further, several rechargeable lithium batteries are connected in series for use in a hybrid car. The operating voltage of the rechargeable lithium battery is 3 times as high as the nickel-cadmium battery or the nickel-metal hydride battery. Further, the rechargeable lithium battery has high energy density per unit weight. For these reasons, the use of rechargeable lithium batteries has increased rapidly.
The rechargeable lithium battery may be fabricated in various shapes, for example, a cylindrical shape or a prismatic shape, which is used for a lithium-ion battery. A lithium polymer battery which has received much attention recently is manufactured in a flexible pouch shape, so that its shape may be variously changed. Further, the lithium polymer battery is excellent in stability and light in weight, so that it is advantageous to achieve the slim and lightweight portable electronic equipment.
Referring to FIG. 1, a conventional pouch-type rechargeable lithium battery 10 includes a battery part 11 and a case 12. The case 12 provides a space 12a for accommodating the battery part 11 therein.
The battery part 11 is made by arranging an anode plate, a separator, and a cathode plate sequentially and winding them in one direction, or by layering a plurality of anode plates, separators, and cathode plates. Each electrode plate of the battery part 11 is electrically connected to an anode or cathode terminal 13 or 14.
One end of each of the anode and cathode terminals 13 and 14 protrudes out of a sealing surface 12b of the case 12. The protruding ends of the anode and cathode terminals 13 and 14 are connected to terminals of a protective circuit board, which is not shown in the drawings.
A sealing tape 15 is wound around a junction of the outer surface of each of the anode and cathode terminals 13 and 14 and the sealing surface 12b so as to prevent a short-circuit between the case 12 and the electrode terminals 13 and 14.
The case 12 does not have a cylindrical or prismatic can-like structure made of a thick gold material, but has a pouched structure which has a metal foil in a middle layer and inner and outer layers attached to opposite surfaces of the metal foil and made of an insulating film. Instead, the pouched case has excellent malleability, so that it may be bent as desired. As described above, the case 12 has the space 12a for accommodating the battery part 11 therein, and the sealing surface 12b is provided on a surface which is thermally fused along the edge of the space 12a. 
FIG. 2 is an enlarged view taken along line I-I of FIG. 1.
Referring to the drawing, the case 12 comprises a composite film having a middle layer 12c and inner and outer layers 12d and 12e. The middle layer 12c is made of metal foil, for example, aluminum foil. The inner and outer layers 12d and 12e comprise insulating films which are attached to inner and outer surfaces of the middle layer 12c to protect the middle layer 12c. 
The battery part 11, having the anode plate 11a, the separator 11c, and the cathode plate 11b sequentially arranged, is housed in the space 12a defined in the case 12. The anode terminal 13 and the cathode terminal 14 extend from the anode plate 11a and the cathode plate 11b, as shown in FIG. 1. Ends of the electrode terminals 13 and 14 are exposed to the outside through the sealing surface 12b of the case 12. The sealing tape 15 surrounds the outer surface of each of the electrode terminals 13 and 14 in the sealing surface 12b. 
The battery part 11 of the pouch-type rechargeable lithium battery 10 constructed as described above is obtained through the following process. First, the anode and cathode terminals 13 and 14 are electrically connected to the anode and cathode plates 11a and 11b. Next, the anode plate 11a, the separator 11c, and the cathode plate 11b are sequentially arranged. In such a state, they are wound in one direction, so that the battery part 11 is completed.
The completed battery part 11 is mounted in the case 12 having the space 12a through a drawing process. During a mounting operation, one end of each of the electrode terminals 13 and 14 is exposed to the outside of the case 12. In such a state, predetermined heat and pressure are applied to the sealing surface 12b of the case 12, so that thermal fusion is executed. Thereby, the pouch-type rechargeable lithium battery 10 is completed. The completed pouch-type rechargeable lithium battery 10 goes through a series of formation processes, including a charging operation, an aging operation, a discharging operation, thus detecting defective batteries, and stabilizing a battery structure.
The prior art dealing with the method of casing the pouch-type rechargeable lithium battery is disclosed in Korean Laid-Open Publication No. 2005-594. According to the cited document, the pouch-type rechargeable lithium battery applies the same positive potential to a metal layer of the case and the anode terminal, thus destroying an inner layer of the case. Due to the destruction of the inner layer, a short-circuit is caused when the cathode terminal and the metal layer of the case contact each other, so that a difference in open circuit voltage is easily detected.
Meanwhile, when a high-power lithium battery is required, like in a hybrid car, several tens or hundreds of pouched batteries, shown in FIGS. 1 and 2, are layered and connected in series so as to provide high voltage.
Since the pouch-type lithium polymer battery comprises a fragile aluminum pouch which is easily bent or curved, the pouch must be protected by a strong case so as to be used for a lengthy period of time. According to the prior art, in order to connect the pouches in series, the anode and cathode terminals of each pouch are connected via a PCB (Printed Circuit Board) having a circuit pattern. The connected pouches are then put into the case.
However, the conventional method of providing the high-power lithium battery by layering the lithium polymer pouches is problematic in that it is difficult to perfectly protect the fragile lithium polymer pouches. Further, the method of layering several pouches and connecting the pouches to the PCB is incomplete, so that it is susceptible to environmental variation, such as external shocks.
Therefore, a method of more firmly and stably layering pouches constituting lithium batteries used as a high-power source, and reliably connecting the pouches in series is required.